Electrophotographic printing is well known and has been widely refined. Using electrophotographic techniques, images are photoelectrically formed on a photoconductive layer mounted on a conductive base. Liquid or dry developer or toner mixtures may be used to develop a requisite image. Liquid toner dispersions for use in the process are formed by dispersing dyes or pigments in natural or synthetic resin materials in a high dielectric constant carrier liquid.
The photoconductive layer is sensitized by electrically charging whereby electrical charges are uniformly distributed over the surface. The photoconductive layer is then exposed by projecting or alternatively by writing an image over the surface with a laser, LED, or the like. The electrical charges on the photoconductive layer are conducted away from the areas exposed to the light with an electrostatic charge remaining in the imaged area. The charged pigment and or dye particles from the liquid toner dispersion contact and adhere to the image area of the plate. The image is then transferred to the desired substrate such as a sheet of paper.
With liquid electrophotography (LEP), the image development process requires that the toner be delivered to the developer at relatively constant concentration. However, usage of toner solids and toner carrier fluid in an LEP process are independent of each other. Toner solid consumption is proportional to the print coverage on the page and the number of pages printed. Whereas, toner carrier consumption is independent of print coverage and only a function of the number of pages printed. These characteristics result in toner concentration decreasing for above average page coverage and increasing for below average page coverage. Additionally, toner solids will settle out from the toner carrier over time.
By measuring both the volume and concentration, user feedback can always be made available as to the percentage of usable toner remaining. Process parameters can be changed to dynamically compensate for toner concentration changes to enhance print characteristics over varying toner concentration ratios. Toner replenishment methods can also be initialized from the concentration measurement. Thus, there exists a need to monitor both the toner concentration and the toner volume in the toner reservoir.